1. Field of the Invention
The subject invention relates to compositions which are used as grinding or dispersing agents for pigments. The compositions are essentially copolymers based upon acrylic acid, methacrylic acid, and sulfonic acid group containing monomers such as 2-acrylamido-2-methylpropanesulfonic acid.
2. Description of the Prior Art
U.S. Pat. No. 3,898,037 describes copolymers of (meth)acrylicamidoalkylsulfonic acid and acrylic acid or methacrylic acid which are used in small quantities (up to approximately 100 ppm) as corrosion inhibitors. The copolymers may be prepared by copolymerization of monomer mixtures in solvent mixtures such as water and isopropanol.
A process for the preparation of polymers of acrylic acid or methacrylic acid or their mixtures is known from German Pat. No. 27 57 329 where the monomers are polymerized in isopropanol or mixtures of water and isopropanol which contain at least 40 percent by weight of isopropanol. The polymerization is carried out with the aid of free radical polymerization initiators at temperatures of 120.degree. C. to 200.degree. C. under pressure. The polymers are used as dispersing agents in the preparation of aqueous pigment slurries for paper sizes. The viscosity of aqueous pigment slurries which contain a polyacrylate as dispersing agent is temperature dependent, decreasing with increased temperature. Even though the polymers are good dispersing agents, they are not as well suited as grinding agents for the preparation of aqueous pigment slurries. This is because the grinding takes place at a temperature range in which the viscosity of the pigment slurry is very low (compared with that at room temperature) so that lower than desired shear forces occur during the grinding process. As a result, the pigment size is not effectively reduced.
Improved grinding and dispersing agents for the preparation of highly concentrated aqueous slurries of pigments for paper sizes are needed. They must be effective and result in highly concentrated aqueous pigment slurries, the viscosity of which is reduced less by an increase in temperature than is the case in aqueous pigment slurries which are prepared with the aid of known agents.